Uranium dioxide-based crystals and methods of fabrication

ABSTRACT

A method of synthesizing uranium dioxide crystals. The method of synthesizing includes combining a uranium-based feedstock with a mineralizer solution. The uranium-based feedstock is selected from uranium dioxide, uranium tetrafluoride, uranium tetrachloride, triuranium octoxide, and uranium trioxide. The feedstock and mineralizer solution are pressurized, and then a thermal gradient is applied thereto such that a first portion of the feedstock and the mineralizer solution is heated to a temperature that is greater than a temperature of a second portion of the feedstock and the mineralizer solution. The uranium nutrient enters the mineralizer solution from the feedstock in the first portion and uranium nutrient precipitates to spontaneously form crystals in the second portion.

Pursuant to 37 C.F.R. § 1.78(a)(4), this application claims the benefitof and priority to Provisional Application Ser. No. 62/139,993, filedMar. 30, 2015, which is expressly incorporated herein by reference.

RIGHTS OF THE GOVERNMENT

This invention was made with government support under DMR-0907395awarded by the National Science Foundation. The Government has certainrights in the invention.

The invention described herein may be manufactured and used by or forthe Government of the United States for all governmental purposeswithout the payment of any royalty.

FIELD OF THE INVENTION

The present invention relates generally to uranium-based crystals and,more particularly, to methods of synthesizing crystals thereof.

BACKGROUND OF THE INVENTION

Neutron detection has a range of applications, including nuclear reactormonitoring, materials science research, nuclear material detection, andnuclear material forensics. Scintillators, materials that absorb energyfrom incoming radiation and emit photons when the scintillator returnsto its original energy state, are used in one method of neutrondetection. Conventional scintillators, which are generally inorganiccrystals, are hygroscopic and require protection from atmosphericconditions to avoid degradation. Therefore, materials containing uraniumoxide or actinide phosphate, such as, RbUPO₄F₂ and CsUPO₄F₂, have beenconsidered promising as state-of-the-art scintillators because of theircharacteristic high density, optical clarity, and stability when exposedto atmospheric conditions. Yet, synthesis of single crystals of thesenew materials has, to date, been unsuccessful.

Moreover, certain uranium oxide or actinide phosphate crystals,particularly RbUPO₄F₂ and CsUPO₄F₂, have the potential to incorporatelarge quantities of radionuclides into their crystal structures.In-fact, it has been hypothesized that the structure of these twomaterials may lend itself to the replacement of the rubidium or cesiumatomic sites with cesium-137 or strontium-90 radioisotopes. Byincorporating these radioisotopes directly into the crystal structure,it is believed that resultant materials may be radiation-damageresistant solid to contain radioactive wastes without the threat ofleaking or degradation over time. Again, as noted above, synthesis ofthese single crystals of actinide phosphate has, to date, beenunsuccessful.

Conventional radioactive waste undergoes a vitrification process, whichincorporates the waste into a glass structure. A borosilicate glassmaterial is commonly used for this process, but is largely inadequatebecause of a lack of stability when exposed to radiation for extendedperiods. When the structure of the glass material become unstable, theglass structure is less effective in containing radioactive wastes. Somehave hypothesized that phosphate structures of RbUPO₄F₂ and CsUPO₄F₂have the potential to render such radioactive waste stable with regardsto temperature and chemical exposure. Similarly, with the incorporationof the uranium in the structure, it also makes it extremely likely thatthese materials will be stable when exposed to radiation for extendedperiods. With a large amount of nuclear wastes produced each year bymedical, industrial, and military processes, these materials would filla large void that exists in current radioactive waste storage anddisposal technology.

Finally, some uranium based single crystals may be useful as a nuclearfuel in a molten salt reactor process.

Uranium-based crystals may, therefore, be used for a plurality ofreasons, not limited to those herein. Yet, to date, bulk substrate ofsingle uranium-based crystals is difficult to achieve. This difficultylies in that, traditionally, high temperatures (in excess of 1000° C.)are needed for crystal growth: that is, in excess of 3000° C. for askull melting technique and in excess of 1000° C. for both flux growthand chemical vapor transport.

Thus, there remains a need for methods of growing single, bulk substrateof uranium-based crystals of high purity, significant size, and highquality.

SUMMARY OF THE INVENTION

The present invention overcomes the foregoing problems and othershortcomings, drawbacks, and challenges of conventional crystal growth.While the invention will be described in connection with certainembodiments, it will be understood that the invention is not limited tothese embodiments. To the contrary, this invention includes allalternatives, modifications, and equivalents as may be included withinthe spirit and scope of the present invention.

According to an embodiment of the present invention, a method ofsynthesizing uranium dioxide crystals includes combining a uranium-basedfeedstock with a mineralizer solution. The uranium-based feedstock isselected from uranium dioxide, uranium tetrafluoride, uraniumtetrachloride, triuranium octoxide, and uranium trioxide. The feedstockand mineralizer solution are pressurized, and then a thermal gradient isapplied thereto such that a first portion of the feedstock and themineralizer solution is heated to a temperature that is greater than atemperature of a second portion of the feedstock and the mineralizersolution. The uranium nutrient enters the mineralizer solution from thefeedstock in the first portion and uranium nutrient precipitates tospontaneously form crystals in the second portion.

Other embodiments of the present invention include a method ofsynthesizing uranium dioxide crystals, which includes combining auranium-based feedstock with a mineralizer solution. The uranium-basedfeedstock is selected from uranium dioxide, uranium tetrafluoride,uranium tetrachloride, triuranium octoxide, and uranium trioxide. A seedcrystal is added to the feedstock and the mineralizer solution, whichare then pressurized. A thermal gradient is applied to the feedstock,mineralizer solution, and seed crystal such that a first portion isheated to a temperature that is greater than a temperature of a secondportion. The uranium nutrient enters the mineralizer solution from thefeedstock in the first portion and uranium nutrient precipitates tospontaneously form crystals on the seed crystal in the second portion.

Still another embodiment of the present invention includes a method ofsynthesizing uranium dioxide crystals and includes combining auranium-based feedstock with a mineralizer solution, the uranium-basedfeedstock. The uranium-based feed stock is selected from uraniumdioxide, uranium tetrafluoride, uranium tetrachloride, triuraniumoctoxide, and uranium trioxide. The uranium-based feedstock and themineralizer solution are placed in an ampoule and pressurized. A thermalgradient is applied to the feedstock and mineralizer solution in theampoule such that a first portion is heated to a temperature that isgreater than a temperature of a second portion. The uranium nutriententers the mineralizer solution from the feedstock in the first portionand uranium nutrient precipitates to spontaneously form crystals in thesecond portion.

Objects, advantages, and novel features of the invention will be setforth in part in the description which follows, and in part will becomeapparent to those skilled in the art upon examination of the followingor may be learned by practice of the invention. The objects andadvantages of the invention may be realized and attained by means of theinstrumentalities and combinations particularly pointed out in theappended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of this specification, illustrate embodiments of the presentinvention and, together with a general description of the inventiongiven above, and the detailed description of the embodiments givenbelow, serve to explain the principles of the present invention.

FIG. 1 is a flowchart illustrating a method of synthesizinguranium-based crystals in accordance with an embodiment of the presentinvention.

FIG. 2 is a side elevational view of an autoclave, shown incross-section, suitable for performing the method of FIG. 1 according toembodiments of the present invention.

FIG. 2A is a side elevational view of an autoclave, shown incross-section, suitable for performing the method of FIG. 1 according toanother embodiment of the present invention.

FIG. 3 is a side elevational view of a seed rack ladder suitable for usein synthesizing uranium oxide crystals in accordance with someembodiments of the present invention.

FIG. 4 is a flowchart illustrating a method of synthesizinguranium-based seed crystals in accordance with another embodiment of thepresent invention.

FIG. 5 is a side elevational view of an autoclave, shown incross-section, suitable for performing the method of FIG. 4 according toembodiments of the present invention.

FIG. 6 is a side elevational view of an exemplary ampoule suitable foruse in synthesizing uranium oxide crystals in accordance with someembodiments of the present invention.

FIGS. 7A-7D are crystal structure representations of RbUPO₄F₂, CsUPO₄F₂,Rb₇U₆F₃₁, and RbUF₅ crystals, respectively, synthesized according to atleast one embodiment of the present invention.

It should be understood that the appended drawings are not necessarilyto scale, presenting a somewhat simplified representation of variousfeatures illustrative of the basic principles of the invention. Thespecific design features of the sequence of operations as disclosedherein, including, for example, specific dimensions, orientations,locations, and shapes of various illustrated components, will bedetermined in part by the particular intended application and useenvironment. Certain features of the illustrated embodiments have beenenlarged or distorted relative to others to facilitate visualization andclear understanding. In particular, thin features may be thickened, forexample, for clarity or illustration.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to the figures, and in particular to FIG. 1, a flowchartillustrating a method 50 of synthesizing single crystals according to anembodiment of the present invention is described. In Block 52, a chamber54 within a pressurizable reaction device 56 (FIG. 2) is prepared with afeedstock and a mineralizer solution (collectively illustrated assolution 57 in FIG. 2A).

Composition of the feedstock and the mineralizer solution depend, inpart, of the desired crystal yielded. The feedstock may be powdered orpolycrystalline and provide nutrient for crystal growth. Foruranium-based crystals, uranium dioxide (UO₂), uranium tetrafluoride(UF₄), uranium tetrachloride (UCl₄), triuranium octoxide (U₃O₈), oruranium trioxide (UO₃) may be used. The mineralizer solution, generallyused for dissolution of nutrient, formation of spuriously nucleatedsingle crystals, or both, may be generally comprised of an alkalihydroxides, ammonium hydroxide, alkali halides, alkali carbonates,alkali fluorides, and mixtures thereof. Mineralizer solutions haveconcentrations ranging from about 0.1 M to about 30 M.

TABLE 1 SEED CRYSTAL FEED- MINERALIZER CRYSTAL (if any) STOCK SOLUTIONUO₂ UO₂ UO₂ CsF CaF₂ UF₄ Alkali hydroxides UCl₄ Ammonium hydroxide U₃O₈Alkali halides UO₃ Alkali carbonates RbF with RbOH RbUPO₄F₂ UO₂ RbH₂PO₄with RbF CsUPO₄F₂ UO₂ CsH₂PO₄ with CsF Rb₇U₆F₃₁ UO₂ RbF RbUF₅ UF₄ RbF +RbOH UCl₄ RbF + HF

The exemplary pressurizable reaction device 56 illustrated in FIG. 2 isan autoclave; however, those skilled in the art having the benefit ofthe disclosure provided herein would readily appreciate that theillustrated structure is nonlimiting. The autoclave 56 includes a wall58 enclosing the chamber 54, which may be separated into upper and lowerregions 54 a, 54 b by a baffle 60. The baffle 60 may be constructed fromany inert material, for example, a precious metal, and includes anopening 62 therein having a diameter, d₁, selected to permit fluidcommunication therethrough ranging from about 15% to about 45%. In thisway, the baffle 60 permits fluidic communication between the upper andlower regions 54 a, 54 b of the chamber 54 while maintaining theseregions 54 a, 54 b as separate. Although the baffle 60 is illustrated ashaving a single opening 62, it would be readily understood that morethan one opening may be used. In-fact, according to some embodiments ofthe present invention, the baffle 60 may be porous or comprise a meshmaterial, for example.

The chamber 54 is accessible through an open end 64, into which a plug66 and seal 68 may be inserted before pressurizing the chamber 54 andsecured with a locking collar.

Externally, heaters 72, 74 (two are shown) at least partially surroundthe wall 58 of the autoclave 56, each corresponding to a respective oneof the upper and lower chambers 54 a, 54 b. The heaters 72, 74 areoperably coupled to a controller 76, which may be configured to operablycontrol the heaters 72, 74 such that the upper chamber 54 a may beheated to a temperature that is different from a temperature of thelower chamber 54 b. Said another way, the heaters 72, 74 may be operatedso as to form a temperature gradient between the upper and lowerchambers 54 a, 54 b. According to embodiments of the present invention,and as described in great detail below, the temperature gradientvariation may range from about 2° C. to about 80° C.

The heaters 72, 74 may have any suitable structure, form, or number.Particularly, and as shown, band heaters 72, 74 are used tocircumferentially surround the 58 and chamber 54 therein. Otherconstructions and methods may be used, so long as a temperaturedifference exists along a longitudinal axis 77 of the chamber 54 of theautoclave 56.

Referring again to FIG. 1, with reference to FIG. 2A, and with thefeedstock and mineralizer solution (collectively illustrated as liquid57) prepared within the chamber 54, a seed crystal 80 may then besuspended within the upper chamber 54 a (Block 78). The crystalsuspension 82 may include wires, clamps, and woven wire mesh constructedfrom an inert material, such as a precious metal.

If necessary, although not shown, de-ionized water may be added to thechamber 54 such that a total volume of solution 57 and water occupiesabout 40% to about 95% of the chamber's internal volume.

Continuing with FIGS. 1 and 2, the chamber 54 of the autoclave 56 maythen be sealed, pressurized (for example, 25,000 psi, but generallyranging from about 10,000 psi to about 30,000 psi), and heated (Block84). In Block 86, a temperature gradient is formed along thelongitudinal axis 77 of the chamber 54. In that the lower chamber 54 bmay be heated to a temperature greater than about 600° C. This hightemperature causes a partial amount of the uranium nutrient/feedstock toenter the mineralizer solution. Concurrently, the upper chamber 54 a maybe heated to a temperature greater than about 550° C. but less than thetemperature of the lower chamber 54 b. At the lower temperature, thesolubility of nutrient in the mineralizer solution is reduced and,resultantly, nutrient will precipitate out of solution and spontaneouslygrow crystals onto the seed crystal 80 (FIG. 2A). More generally, themaximum temperature may range from about 400° C. to about 750° C., withthe thermal gradient ranging from about 2° C. to about 80° C.

Heating and crystallization continue (“No” branch of decision block 88)until a final crystal is achieved and having one or more of a desiredpurity, a desired quality, and a desired size. While thesecharacteristics of the final crystal are at least partially dependent onreaction duration, generally crystal growth continues for about 7 daysto about 90 days.

When the desired growth is achieved (“Yes” branch of decision block 88),the process ends, the heat and pressure are removed from the chamber 54such that crystal may be retrieved.

According to some alternative embodiments, the thermal gradient need notbe applied nor maintained. Instead, crystal growth may be foundfavorable using an isothermal temperature.

According to some embodiments of the present invention, the use of oneor more seed crystal 80 may be required or desired. In that regard, andwith reference to FIG. 3, a baffle-based seed ladder 90 is shown. Thebaffle-based seed ladder 90 includes a baffle portion 92 and a ladderportion 94 and, thus, may comprise a unitary construction or,alternatively, may be separately constructed and joined together. As wasnoted above, the construction may include any inert material, forexample, precious metals.

The baffle portion 94 includes an opening 96 within a main body 98having a diameter, d₂, selected to permit fluid communicationtherethrough ranging from about 15% to about 45% and so as to permitfluidic communication between the upper and lower regions 54 a, 54 b(FIG. 2) of the chamber 54 (FIG. 2) while maintaining these regions 54a, 54 b (FIG. 2) as separate.

The ladder portion 94 includes a one or more rungs 100 (three rungs 100are shown) extending from vertical supports 102. Seed crystals 80 (twoseed crystals 80 are shown) are positioned between adjacent ones of therungs 100 by at least one suspension 82, which may be similar to thesuspensions discussed in detail above.

Use of the baffle-based seed ladder 90 may provide the benefit ofgrowing more than one crystal at a time in accordance with embodimentsof the present invention as described in detail here.

Turning now to FIGS. 4 and 5, a method of forming crystals according toanother embodiment of the present invention is shown. In Block 112, anampoule 114 configured to be positioned within a chamber 116 of apressurizable reaction device 118 is prepared with a feedstock and amineralizer solution. As described previously, the composition of thefeedstock and the mineralizer solution depends, in part, of the desiredcrystal yielded and may be selected in accordance with the parametersset forth above. Again, mineralizer solutions may have concentrationsranging from about 0.1 M to about 30 M.

The exemplary pressurizable reaction device 118 illustrated in FIG. 5 issimilar to the autoclave 56 of FIG. 2; however, those skilled in the arthaving the benefit of the disclosure provided herein would readilyappreciate that the illustrated structure is nonlimiting. Here, thedevice 118 includes a wall 120 enclosing the chamber 116. The chamber116 is accessible through an open end 122, into which a plug 124 andseal 126 may be inserted before pressurizing the chamber 116 and securedwith a locking collar 128.

Externally, heaters 130, 132 (two are shown), similar to those describedabove, at least partially surround the wall 120 of the device 118. Theheaters 130, 132 may be operably controlled by a controller 134 such atemperature gradient is created along a longitudinal axis 77 (FIG. 2) ofthe chamber 116. According to embodiments of the present invention, andas described in great detail below, the temperature gradient variationmay range from about 2° C. to about 80° C.

The ampoule 114 may be constructed of a precious metal (silver, gold,platinum, or palladium, for example) and, according to some embodimentsof the present invention, may comprise a metal tubing, such as thosecommercially-available from by Refining Systems, Inc. (Las Vegas, Nev.)and having one end welded or otherwise closed to retain the feedstockand the mineralizer solution therein.

Referring again to Block 112, the feedstock and the mineralizer solutionare added to the ampoule 114 until a combined total of the feedstock andmineralizer solution within the ampoule is set to occupy a majoritypercentage (ranging from about 40% to about 90%) of the ampoule's totalvolume. The ampoule 114 may then be sealed (for example, by welding anyopen end) and is positioned within the chamber 116 of the device 118 ofFIG. 5 (Block 136). If necessary, although not shown, de-ionized watermay be added to the chamber 116 such that a total volume of ampoule 114and water occupies about 65% to about 90% of the chamber's internalvolume.

Continuing with FIGS. 4 and 5, the chamber 116 of the device 118 maythen be sealed, pressurized (for example, 25,000 psi, but generallyranging from about 10,000 psi to about 30,000 psi), and heated (Block138). In Block 140, a temperature gradient is formed along thelongitudinal axis 77 (FIG. 2) of the chamber 116, which may generallycoincide with a longitudinal axis (not shown) of the ampoule 114. Inthat regard, the ampoule 114 will have a lower region 114 a heated to atemperature that is greater than a temperature of an upper region 114 b.It should be readily appreciated that the terms “lower” and “upper” aremerely used as directional reference herein with respect to FIG. 5 andshould not be considered to be limiting.

According to some embodiments, the highest temperature of the ampoule114 at the lower region 114 a will be greater than about 600° C. At thishigh temperature, uranium nutrient/feedstock enters the mineralizersolution. The upper region 114 b may then heated to a temperaturegreater than about 550° C. but less than the temperature of the lowerchamber 54 b. At the lower temperature, the solubility of nutrient inthe mineralizer solution is reduced and, resultantly, nutrient willprecipitate out of solution and spontaneously form spontaneouslycrystals on an inner wall (not shown) of the ampoule 114). Moregenerally, the maximum temperature may range from about 400° C. to about750° C., with the thermal gradient ranging from about 2° C. to about 80°C.

Heating and crystallization continue (“No” branch of decision block 142)until a desired growth is achieved. While the final size of the crystalis dependent on reaction duration, generally crystal growth continuesfor about 7 days to about 90 days.

When the desired growth is achieved (“Yes” branch of decision block142), a decision is made as to whether larger crystals are desired(Decision block 144). If larger crystals are desired (“Yes” branch ofdecision block 144), then heat and pressure are removed from the chamber116, the ampoule 114 opened, and a small crystal may be extracted fromthe inner wall of the ampoule 114 (Block 146). The small, extractedcrystal may then be used as a seed crystal in the method 50 (FIG. 1)described above. Otherwise (“No” branch of decision block 144), theprocess ends, the heat and pressure are removed from the chamber 116 andthe ampoule 114 opened such that crystals may be retrieved.

Similar to the alternate embodiment described above, an ampoule 150,used in accordance with methods described herein, may further comprise abaffle 152, with or without a seed ladder 154, the latter of which isshown in FIG. 6 (the ampoule 150 being in phantom). The baffle 152 withladder 154 may comprise a unitary construction of an inert material(such as a precious metal) or, alternatively, may be separatelyconstructed and joined together. As was noted above, the constructionmay include any inert material, for example, precious metals.

The baffle 152 includes an opening 156 within a main body 158 having adiameter selected to permit fluid communication therethrough rangingfrom about 15% to about 45% and so as to permit fluidic communicationbetween the upper and lower regions 150 b, 150 a of the ampoule 150while maintaining these regions 150 b, 150 a as separate.

The seed ladder 154 includes a one or more rungs 160 (three rungs 160are shown) extending from vertical supports 162. Seed crystals 80 (twoseed crystals 80 are shown) are positioned between adjacent ones of therungs 160 by at least one suspension 164, which may be similar to thesuspensions discussed in detail above. In this way, more than one seedcrystal 80 may be used for growing crystals.

The following examples illustrate particular properties and advantagesof some of the embodiments of the present invention. Furthermore, theseare examples of reduction to practice of the present invention andconfirmation that the principles described in the present invention aretherefore valid but should not be construed as in any way limiting thescope of the invention.

Example 1

Uranium dioxide crystals were synthesized by placing a powdered orpolycrystalline nutrient/feedstock of UO₂ in a lower heating zone of asilver ampoule. A seed crystal of UO₂ was suspended in the upper heatingzone of the ampoule on a seed rack, similar to the embodimentillustrated in FIG. 6. A 6 M cesium fluoride mineralizer solution wasadded to fill 70% of the ampoule. A baffle comprising a precious metalwas positioned between the lower heating zone and the upper heating zoneof the ampoule. The ampoule was then welded shut and placed within areaction chamber with water (enough to the 75% to 80% fill level) of anautoclave (similar to the embodiment illustrated in FIG. 5).

Band heaters on the autoclave were operated to maintain the lowerheating zone at about 600° C. and the upper heating zone at about 550°C. A growth pressure of about 25,000 psi was maintained. Growthcontinued for 14 days.

New growth of UO₂ crystalline material was deposited on the seedcrystal, which enlarged the seed crystal by approximately 1 mm in eachdimension.

Example 2

Uranium dioxide crystals were synthesized by placing a powdered orpolycrystalline nutrient/feedstock of UO₂ in a lower heating zone of asilver ampoule. A 9 M cesium fluoride mineralizer solution was added tofill 70% of the ampoule. A baffle comprising a precious metal waspositioned between the lower heating zone and the upper heating zone ofthe ampoule. The ampoule was then welded shut and placed within areaction chamber with water (enough to the 75% to 80% fill level) of anautoclave (similar to the embodiment illustrated in FIG. 5).

Band heaters on the autoclave were operated to maintain the lowerheating zone at about 650° C. and the upper heating zone at about 600°C. A growth pressure of about 25,000 psi was maintained. Growthcontinued for 7 days.

Resultant and spontaneously nucleated UO₂ crystals were approximately0.25 mm in size.

Example 3

Uranium dioxide crystals were synthesized using a CaF₂ seed crystal. Theorientation of the seed crystal may vary, but may generally be (100) and(111) and, preferably, (110). Additionally, a miscut of ranging from 2°to about 4° may be used where the nominal orientation is (100), (111),or (110). Synthesis occurred according to the method of Example 1;however, no thermal gradient was applied—that, is, the entire reactionwas maintained at an isothermal temperature of 650° C. Moreover, as thereaction was isothermal, no baffle was used.

The CaF₂ seed crystal was suspended slightly above the UO₂ feedstocksuch that UO₂ nutrients could dissolve into the feedstock priorprecipitation onto the seed crystal.

Example 4

Uranium dioxide crystals were again synthesized using a CaF₂ seedcrystal and with a thermal gradient according to embodiments of thepresent invention. As was described in Example 3, the orientation of theseed crystal was variable, but generally (100) and (111) and,preferably, (110). Additionally, a miscut of ranging from 2° to about 4°may be used where the nominal orientation is (100), (111), or (110).Synthesis occurred according to the method of Example 1 with a thermalgradient of 60°.

The CaF₂ seed crystal was suspended slightly above the UO₂ feedstocksuch that UO₂ nutrients could dissolve into the feedstock priorprecipitation onto the seed crystal.

Relatively large UO₂ crystals were formed in a relatively short amountof time under these conditions.

Example 5

Rubidium uranium fluorophosphate crystals were successfully synthesizedusing a hydrothermal growth technique according to a method of thepresent invention. A mineralizer solution comprising 3.15 M RbH₂PO₄ and6.3 M RbF was prepared and placed with a uranium feedstock in anampoule. The total volume of the mineralizer solution with the feedstockwas between 60% and 70% of the total ampoule volume.

The ampoule was welded shut and placed into an autoclave, similar to theembodiment illustrated in FIG. 2. Deionized water was added such thatthe total volume of the ampoule with the deionized water occupied 65% to75% of the autoclave reaction chamber volume. The autoclave waspressurized to about 25,000 psi, which was maintained throughout thereaction time.

Heat was applied such that the lower region of the ampoule wasmaintained at about 650° C. while the upper region of the ampoule wasmaintained at about 600° C.

Resultant crystals were analyzed using a XtaLab Mini™ (Rigaku Corp.,Tokyo, Japan) operating at room temperature with Mo kα, λ=0.71073, 50kV, 12 mA, and 0.6 kW. Relevant crystal information is shown in Table 2,below.

TABLE 2 Crystal Data for RbUPO₄F₂ and CsUPO₄F₂ Chemical Formula CsVPO₄F₂RbUPO₄F₂ Crystal System Monoclinic Monoclinic Space Group P2₁/m (no. 11)P2₁/m (no. 11) a (Å) 6.7891 (14) 6.6770 (13) b (Å) 5.9910 (12) 5.9420(12) c (Å) 7.6040 (15) 7.3470 (15) α (°) 90.00 90.00 β (°) 115.73 (3) 114.07 (3)  γ (°) 90.00 90.00 Z 2 2

Example 6

Cesium uranium fluorophosphate crystals were successfully synthesizedusing a hydrothermal growth technique similar to Example 5 with themineralizer solution comprising 3.15 M CsH₂PO₄ and 6.3 M CsF wasprepared and placed with a uranium feedstock in an ampoule. Resultantcrystal information is shown in Table 2, above.

Exemplary crystal structure representations for RbUPO₄F₂ and CsUPO₄F₂are shown in FIGS. 7A and 7B, respectively.

Example 7

Rubidium uranium fluoride crystals (Rb₇U₆F₃₁) were successfullysynthesized using a growth technique similar to Example 5. In thatregard, a mineralizer solution comprising 2 M RbF and 1 M RbOH wasprepared and placed with a uranium feedstock in an ampoule. Resultantcrystal information is shown in Table 3, below.

Rubidium uranium fluoride crystals (RbUF₅) were successfully synthesizedusing a growth technique similar to Example 5. In that regard, amineralizer solution comprising 0.1 M RbF and 0.05 M RbOH was preparedand placed with a uranium feedstock in an ampoule. Resultant crystalinformation is shown in Table 3, below.

Exemplary crystal structure representations for Rb₇U₆F₃₁ and RbUF₅ areshown in FIGS. 7C and 7D, respectively.

TABLE 3 Crystal Data for Rb₇U₆F₃₁ and RbUF₅ Chemical Formula Rb₇U₆F₃₁RbUF₅ Crystal System Rhombohedral Monoclinic Space Group R-3 (no. 148)P2₁/c (no. 14) a (Å) 15.246 (2) 8.2690 (17) b (Å) 15.426 (2) 13.747 (3) c (Å) 10.715 (2) 8.3560 (17) α (°) 90.00 90.00 β (°) 90.00 102.34 (3)  γ(°) 120.00 90.00 Z 3 8

While the present invention has been illustrated by a description of oneor more embodiments thereof and while these embodiments have beendescribed in considerable detail, they are not intended to restrict orin any way limit the scope of the appended claims to such detail.Additional advantages and modifications will readily appear to thoseskilled in the art. The invention in its broader aspects is thereforenot limited to the specific details, representative apparatus andmethod, and illustrative examples shown and described. Accordingly,departures may be made from such details without departing from thescope of the general inventive concept.

What is claimed is:
 1. A method of synthesizing uranium dioxidecrystals, the method comprising: combining a uranium-based feedstockwith a mineralizer solution, the uranium-based feedstock selected fromthe group consisting of uranium dioxide, uranium tetrafluoride, uraniumtetrachloride, triuranium octoxide, and uranium trioxide; pressurizingthe feedstock and the mineralizer solution; and applying a thermalgradient to the pressurized feedstock and the mineralizer solution suchthat a first portion of the feedstock and the mineralizer solution isheated to a temperature that is greater than a temperature of a secondportion of the feedstock and the mineralizer solution, wherein uraniumnutrient enters the mineralizer solution from the feedstock in the firstportion and uranium nutrient precipitates to spontaneously form crystalsin the second portion.
 2. The method of claim 1, wherein the mineralizersolution is selected from the group consisting of cesium fluoride,alkali hydroxides, ammonium hydroxides, alkali halides, alkalicarbonates, and rubidium fluoride with rubidium hydroxide.
 3. The methodof claim 2, wherein a concentration of the mineralizer solution rangesfrom about 0.1 M to about 30 M.
 4. The method of claim 1, furthercomprising: adding a seed crystal to the feedstock and the mineralizersolution.
 5. The method of claim 4, wherein the seed crystal comprisesuranium dioxide or calcium difluoride.
 6. The method of claim 1, furthercomprising: placing the feedstock with the mineralizer solution in anampoule before pressurizing.
 7. The method of claim 6, furthercomprising: adding a seed crystal with the feedstock and the mineralizersolution in the ampoule.
 8. The method of claim 6, positioning a bafflein the ampoule and between the first and second portions.
 9. A method ofsynthesizing uranium dioxide crystals, the method comprising: combininga uranium-based feedstock with a mineralizer solution, the uranium-basedfeedstock selected from the group consisting of uranium dioxide, uraniumtetrafluoride, uranium tetrachloride, triuranium octoxide, and uraniumtrioxide; adding a seed crystal to the uranium-based feedstock and themineralizer solution; pressurizing the uranium-based feedstock and themineralizer solution with the seed crystal; and applying a thermalgradient to the pressurized uranium-based feedstock, the mineralizersolution, and the seed crystal such that a first portion of theuranium-based feedstock and the mineralizer solution is heated to atemperature that is greater than a temperature of a second portion ofthe feedstock and the mineralizer solution, wherein uranium nutriententers the mineralizer solution from the feedstock in the first portionand uranium nutrient precipitates to spontaneously form crystals on theseed crystal in the second portion.
 10. The method of claim 9, whereinthe seed crystal comprises uranium dioxide or calcium difluoride. 11.The method of claim 9, wherein the mineralizer solution is selected fromthe group consisting of cesium fluoride, alkali hydroxides, ammoniumhydroxides, alkali halides, alkali carbonates, and rubidium fluoridewith rubidium hydroxide.
 12. The method of claim 9, further comprising:placing the feedstock and the mineralizer solution with the seed crystalin an ampoule before pressurizing.
 13. The method of claim 12,positioning a baffle in the ampoule and between the first and secondportions.
 14. A method of synthesizing uranium dioxide crystals, themethod comprising: combining a uranium-based feedstock with amineralizer solution, the uranium-based feedstock selected from thegroup consisting of uranium dioxide, uranium tetrafluoride, uraniumtetrachloride, triuranium octoxide, and uranium trioxide; placing thefeedstock with the mineralizer solution in an ampoule; pressurizing thefeedstock and the mineralizer solution in the ampoule; and applying athermal gradient to the pressurized feedstock and the mineralizersolution such that a first portion of the feedstock and the mineralizersolution is heated to a temperature that is greater than a temperatureof a second portion of the feedstock and the mineralizer solution,wherein uranium nutrient enters the mineralizer solution from thefeedstock in the first portion and uranium nutrient precipitates tospontaneously form crystals in the second portion.
 15. The method ofclaim 14, further comprising: adding a seed crystal with the feedstockand the mineralizer solution in the ampoule.
 16. The method of claim 15,positioning a baffle in the ampoule and between the first and secondportions.